1. Field of the Invention
The present invention relates to a synchronous circuit which generates a frame synchronizing signal by detecting a detection synchronizing signal generated from a reproduction signal, in a detecting window having a prescribed width, and to an optical disk reproduction device having the synchronous circuit.
2. Description of the Related Art
Japanese Patent Laying-Open No. 2-306472 discloses a synchronizing circuit which detects a synchronizing signal in a detecting window having a prescribed time width to produce a frame synchronizing signal. The synchronizing circuit increases, if a synchronizing signal is not detected, the width of the detecting window at a next detection timing to achieve a higher possibility of detecting a synchronizing signal at the next detection timing.
An optical disk reproduction device which reproduces a signal read from an optical disk by demodulating the signal is provided with an error correction circuit which corrects an error in data after demodulation. The ability to correct an error of the error correction circuit is growing. For example, in the specification of the digital video disk (DVD), even if there is a succession of reproduction errors in 32 frames, the errors can be corrected if the data preceding and following the reproduction errors can be correctly reproduced.
According to the synchronizing circuit disclosed in the Japanese Patent Laying-Open No. 2-306472, if a synchronizing signal is not detected, the width of the detecting window is increased at the next detection timing. However, the increase of the detecting window width enlarges the possibility of mistakenly recognizing, a bit string read accidentally as having the same array as a synchronizing pattern in a reproduction error or the like as the synchronizing pattern. One example of a detection synchronizing signal produced by mistakenly detecting a synchronizing pattern is shown as signal a in FIG. 7Q. Signal .alpha. is not mistakenly detected since signal .alpha. is not present in the detecting window. However, if the width of the detecting window is increased, a frame synchronizing signal based on signal a produced by the mistaken detection may be generated.
In the optical disk reproduction device having the enhanced ability of correcting an error, an error in one frame can be easily corrected. In other words, if one synchronizing signal or a small number of consecutive synchronizing signals is/are not detected, there is no problem about reproduction of data. Therefore, there is no need to increase the width of the detecting window immediately after a synchronizing signal is not detected as in the synchronizing circuit disclosed in the Japanese Patent Laying-Open No. 2-306472. On the other hand, if the width of the detecting window is increased immediately after a synchronizing signal is not detected, the possibility of producing an erroneous synchronization could be increased due to an erroneous detection of a synchronizing pattern.
Generally, increase of the width of the detecting window is advantageous when the position of the detected synchronizing pattern is not at an originally expected normal position, not when a random error is produced as shown in FIG. 3D in which a synchronizing pattern cannot be detected due to a reproduction error or the like. In such a case, synchronizing signals are not consecutively detected as in the case of a burst error shown in FIG. 3E. As a result, errors beyond the correction ability of the error correction circuit are produced resulting in any trouble in reproduction of data.